1. Field of the Invention
The invention relates to a method and a device for generating a positive pressure in a tank for liquefied gas on a refrigerated vehicle with an evaporator for the liquefied gas, in conjunction with which the evaporator is connected in a fluid-conducting fashion to the tank via a line for liquefied gas, and in conjunction with which a valve is arranged in the line; to a cooling system for a refrigerated vehicle and a refrigerated vehicle having at least one refrigerated chamber, a tank for liquefied gas and an evaporator for the evaporation of the liquefied gas and the delivery of cold to the refrigerated chamber, in conjunction with which the evaporator is connected in a fluid-conducting fashion to the tank via a line for liquefied gas, and in conjunction with which a valve is arranged in the line.
2. Related Art
For approximately 30 years, nitrogen has been used for the refrigeration of vehicles with multi-chamber systems. A method of this type is already familiar under the name CryogenTrans (CT). The CT method involves carrying nitrogen in liquid form at low temperature in a vacuum-insulated container on or in the vehicle. As and when cold is required, this nitrogen is drawn off via a pipe and is sprayed directly into the chamber to be refrigerated by the inherent pressure of the medium. The method is particularly simple and is immune to interference. What is more, the refrigerating capacity is always at the same level regardless of the ambient temperature. It is restricted in principle only by the flow capacity of the spray nozzles. As a consequence of this, CT refrigerated goods vehicles, which are used in foodstuffs distribution traffic and naturally have numerous door openings during refrigerated operation, exhibit considerable advantages in respect of the quality of the refrigeration. In particular in the height of the summer, when mechanical refrigeration plants have to struggle with reduced performance of their condensers and with icing-up of their evaporators, the CT method demonstrates its advantages in terms of efficiency, dependability and performance. After opening a door, it takes only seconds for the reference temperature to be achieved once again.
The method also has its disadvantages, however. The consumption of nitrogen is relatively high, because at least some of the gas sprayed into a chamber also escapes again as exhaust gas. If, for example, a frozen food chamber is refrigerated, the temperature of the exhaust gas will be in the order of −30 to −40° C. The fact that a load space requires to be fully ventilated for reasons of safety before being entered is also disadvantageous. An unnecessarily large quantity of warm air enters the load space in this case. Although the renewed reduction in temperature admittedly takes place very rapidly, it consumes more energy and as a result incurs more costs than necessary. The otherwise customary installation of cold retention systems, such as a curtain, is inappropriate in the case of CT refrigerated vehicles, because they would impair the ventilation in a dangerous manner.
EP 0 826 937 describes a refrigeration unit for a chamber to be refrigerated.
EP 1 593 918 relates to an indirect means of refrigeration for refrigerated vehicles, in which a heat exchanger is arranged for the evaporation of low-temperature liquefied gas in a refrigerated chamber.
Liquefied low-temperature nitrogen has a temperature of 77° K under normal pressure. The cold that is stored in this case is present as two components: on the one hand as a component that is liberated during the phase transition from liquid to gaseous at a temperature of 77° K, and on the other hand as a component that absorbs heat in conjunction with heating of the gaseous phase from 77° K up to the exhaust gas temperature. The two components, enthalpy of evaporation and specific heat, are of approximately the same size as a rule.
Previously disclosed is the supply with the help of a motorized pump of the liquefied gas stored in a tank and carried on the vehicle from the tank into an evaporator of a heat exchanger, in order to deliver the stored cold to a refrigerated chamber of the refrigerated vehicle.
Also previously disclosed is the utilization of the vapor pressure created above the liquid phase in the tank for the supply of the liquefied gas into an evaporator arranged above the tank.
A substantial positive pressure must be present inside the tank, however, in order to ensure a reliable mode of operation of the cooling system, for which purpose the temperature inside the tank must be noticeably higher than the boiling temperature of the liquefied gas at ambient pressure. Raising the temperature means that a component of the cold that is stored in the liquefied gas is not utilized. Moreover, cooling shortly after charging the tank with fresh liquefied gas is not yet possible as a rule, because the temperature of the gas at the time of its supply is too cold to be able to assure the generation of pressure inside the tank. The use of heating elements inside the tank is admittedly possible, although this is not optimal in a thermodynamic sense. The use of a motorized pump is admittedly justifiable in a thermodynamic sense, although it is comparatively exposed to interference and comparatively maintenance-intensive.